In the context of dual chamber pacing, a variety of mode switching features have been developed which detect an excessively rapid atrial rhythm and, in response, cause the pacemaker to switch from an atrial synchronized pacing mode, such as DDD, to a non-synchronized mode such as VVI or DDI. Such mode switching features are disclosed in U.S. Pat. No. 5,144,949, by Olson, U.S. Pat. No. 5,318,594, by Limousin et al., U.S. Pat. No. 4,944,298, by Sholder, U.S. Pat. No. 5,292,340, by Crosby et al. and U.S. Pat. No. 4,932,406 by Berkovits, all incorporated herein by reference in their entireties. In such devices, the primary purpose of the mode switch is to prevent the pacemaker from tracking a non-physiologic atrial rate.
It is common in dual chamber pacing with both atrial and ventricular sensing leads for the atrial sensing channel to be blanked after a ventricular event for a specified blanking interval. This is done to avoid oversensing, including far-field sensing of ventricular depolarizations by the atrial sensing lead. The blanking periods can complicate the detection of atrial tachycardia or atrial flutter since the blanking periods can block detection of some atrial events. Further, a dual chamber device with pacing pulse timing optimized to improve patient hemodynamics may have sequential right and left ventricular pacing pulses that are often separated in time by as much as 80 ms. The sequential pacing of such devices increases the atrial blanking period and, thereby, increases the difficulty of detecting AF or atrial flutter.